Steel and metals

The fabrication shop is a tall shed beside the railway, smelling of cutting oil and hot metal, and the shop manager walks the class past a beam line where a saw and drill cut and punch steel sections to the millimeter from a digital model. A welder lays a bead on a column base plate; sparks fly off a grinder. In the yard, a stack of wide-flange beams waits for shipping, already marked with piece numbers. Callum notices that some of the steel is dark gray with mill scale, some is painted red-brown primer and a few pieces are dull silver. 'Bare, primed and hot-dip galvanized,' says Vasić. 'Three answers to the same question: how long do you want it to last, and where?'

Iron and steel

Steel is iron with a small, controlled amount of carbon, usually less than 0.3% in structural steel, plus small additions of manganese, silicon and other elements. Pure iron is soft; a little carbon makes it strong; too much makes it brittle, as in cast iron, with 2 to 4% carbon. Wrought iron, used in nineteenth-century bridges and buildings, had very little carbon but contained fibrous slag.

There are two main routes to steel. In the integrated route, iron ore, coke and limestone are reduced in a blast furnace to make iron, which is refined into steel in a basic oxygen furnace; this route emits about two tons of carbon dioxide for each ton of steel. In the electric arc furnace route, scrap steel is melted by an electric arc and refined. This route uses far less energy and emits far less carbon, especially where electricity is clean, and most structural sections rolled in North America are now made this way with a high recycled content. Plate, sheet and some hollow sections still often come from integrated mills. Emerging processes reduce iron ore with hydrogen instead of coke, and several plants are under construction worldwide.

Steel is shaped by hot rolling into wide-flange (W) sections, channels, angles, plates and bars; by cold forming of thin sheet into studs, joists and deck; and into hollow structural sections (HSS), the rectangular and round tubes beloved of architects for their clean look. In the United States, W sections are usually ASTM A992 with a yield strength of 50 ksi; plates, angles and channels are often ASTM A36 at 36 ksi; HSS are commonly ASTM A500 or the newer A1085.

How steel behaves under load

Steel is strong, stiff and ductile. Under increasing load it first stretches elastically, returning to its original length when unloaded. At the yield point it begins to stretch permanently with little extra load, over a long plateau; then it strain-hardens to an ultimate strength before necking and breaking. This ductility is a great virtue: steel structures give warning before they fail, and connections can share load as parts yield. Concrete and clear wood, by contrast, fail in a brittle way.

Steel's stiffness, with a modulus of about 29 million psi, is the same for every grade; a higher-strength steel lets a member carry more load but does not make it any stiffer. That is why a long-span steel beam is often sized for deflection, and why simply specifying a stronger grade rarely makes a floor bounce less.

Steel and heat

Two thermal properties shape steel detailing. First, steel conducts heat well, about four hundred times better than wood. A steel beam or balcony slab connector passing from inside to outside is a thermal bridge that wastes energy and can cause condensation and frost on the interior. Manufactured thermal break pads and connectors interrupt the path.

Second, steel loses strength in fire. At about 750 °F structural steel begins to lose strength; at about 1,020 °F it retains roughly half its strength at room temperature, enough to collapse under full design load. A severe fire can heat unprotected steel to that temperature within about fifteen to twenty minutes. Codes therefore require protection for steel in most buildings beyond a certain size: spray-applied fire-resistive materials, which are cheap but ugly and must be hidden; intumescent coatings, thin paints that swell into an insulating char in fire, which let steel stay visible; or encasement in gypsum board or concrete.

Steel also expands with temperature, by about 6.5 millionths per °F. The rule for all thermal movement is the same.

Here ΔL is the change in length, α the coefficient of thermal expansion, L the length and ΔT the change in temperature. Metals exposed to sun can be much hotter than the air: a dark metal panel on a south wall can reach 160 °F on a still summer afternoon and fall to -30 °F on a January night.

# Thermal movement of exposed elements on the community hub, from the coldest night to the hottest afternoon
alpha = {"steel": 6.5e-06, "Al alloy": 1.3e-05, "zinc sheet": 1.2e-05, "copper sheet": 9.4e-06, "PVC": 3.3e-05}   # per degree
cases = [  # element, material, length (ft), low and high surface temperatures
    ("steel canopy beam", "steel", 26, -30, 120),
    ("curtain wall mullion", "Al alloy", 14, -30, 120),
    ("zinc standing-seam panel", "zinc sheet", 20, -30, 160),
    ("copper gutter run", "copper sheet", 40, -30, 160),
    ("PVC trim board", "PVC", 12, -30, 160),
]
for name, mat, length, lo, hi in cases:
    dl = alpha[mat] * length * 12 * (hi - lo)
    print(f"{name:<26} {length:>4} ft  moves {dl:5.2f} in")

steel canopy beam            26 ft  moves  0.30 in
curtain wall mullion         14 ft  moves  0.33 in
zinc standing-seam panel     20 ft  moves  0.55 in
copper gutter run            40 ft  moves  0.86 in
PVC trim board               12 ft  moves  0.90 in

The results surprise the students. A 14 ft window mullion moves as much as a steel beam twice its length, and a short PVC trim board moves almost as much as a long copper gutter. Each number becomes a detail. The canopy beam needs a slotted connection at one end so it can slide. The curtain wall mullions need a sliding joint at each floor, usually a sleeve called a stack joint. Zinc and copper sheets are held by sliding clips rather than nailed through, and long gutters need expansion joints. PVC trim, the worst offender, needs gaps at every butt joint and fasteners in oversized holes, or it will buckle in summer and open in winter.

Movement is also why metal cladding panels are rarely fixed rigidly on all four sides. A typical rule is to fix a panel at one point, guide it at others so it can slide, and cover the joints with laps or gaskets so water stays out while the panel moves. Vasić shows a photograph from her consultancy days of a run of flat metal panels bowed like pillows, the result of fixing them tightly at every edge on a west wall.

Corrosion

Iron and steel rust when water and oxygen are both present. Rust is iron oxide, weaker than steel and occupying several times its volume, so it flakes away, exposing fresh steel, and it can burst concrete or masonry around buried steel. Indoors, in heated dry air, bare structural steel hardly corrodes. Outdoors, in a wet or salty environment, unprotected steel can lose a significant amount of metal over a building's life.

Designers protect steel in four ways. Paint systems create a barrier; they work well if the steel is properly cleaned before painting and the paint is maintained. Hot-dip galvanizing dips the fabricated steel into molten zinc, which bonds metallurgically and protects in two ways: as a barrier and sacrificially, because zinc corrodes in preference to steel wherever the coating is scratched. A typical hot-dip coating lasts many decades in a rural or suburban atmosphere, less near salt. Duplex systems, paint over galvanizing, last longer than either alone. Finally, corrosion-resistant metals can replace carbon steel.

Weathering steel, with small additions of copper, chromium and nickel, forms a dense, stable rust layer that slows further corrosion in suitable conditions. ASTM A588 and A847 are typical specifications. It needs alternate wetting and drying to form the patina and must not sit wet, be buried or face de-icing salts; its run-off stains concrete and paving below, so details must collect or direct the water.

Stainless steel contains at least 10.5% chromium, which forms an invisible, self-healing oxide film. Type 304 suits most building exteriors; Type 316, with molybdenum, resists chlorides better near roads treated with salt and by the sea. Stainless steel costs several times as much as carbon steel and is used for fasteners, ties, anchors, railings, flues and exposed fittings rather than main structure.

Galvanic corrosion

When two different metals touch in the presence of water, they form a battery, and the more active metal corrodes faster while the more noble one is protected. The galvanic series ranks metals for typical outdoor conditions.

Practical consequences follow. Copper flashing must not drain onto galvanized steel or aluminum gutters; copper ions in the run-off will corrode them. Steel fasteners in copper roofing will rust quickly; stainless or copper fasteners are used instead. Aluminum windows fixed with plain steel screws corrode at the screws. Where dissimilar metals must meet, they are separated with non-conductive washers, tapes or coatings, and water is kept from bridging them. The rule also explains why galvanizing works: zinc sacrifices itself to protect steel.

Other metals in buildings

Aluminum is light, a third the density of steel, easily extruded into complex shapes and naturally protected by a thin oxide film. It is the usual material for window and curtain wall frames, sunshades, flashing and many cladding panels. Its weaknesses are a modulus a third of steel's, high thermal conductivity and high thermal expansion. Window frames therefore need thermal breaks of polyamide or polyurethane, and long extrusions need sliding joints. It is finished by anodizing, which thickens the oxide, or by baked-on coatings such as PVDF. Primary aluminum from bauxite is energy-intensive, with a large carbon footprint if made with fossil electricity; recycled aluminum uses about a twentieth of the energy, and much Canadian smelting uses hydroelectricity.

Copper has been used on roofs and domes for centuries. It weathers from bright salmon to brown to the familiar green patina over years or decades, depending on climate. It is soft, workable and very durable, but expensive. Zinc, sold as rolled sheet often alloyed with titanium and copper, forms a blue-gray patina and is used for standing-seam roofs and cladding; it needs a ventilated underside, because moisture trapped against its back side prevents the protective layer forming and causes corrosion. Lead, once standard for flashing, is now avoided in most applications because of its toxicity, especially near drinking water and children.

Light-gauge steel framing, studs and joists cold-formed from galvanized sheet, has become common for non-loadbearing interior walls, backup walls behind cladding and mid-rise loadbearing frames. Its galvanized coating protects it during construction and in dry walls, but it must be kept out of persistently wet assemblies.

Exercise 5: steel and metals

• Visit a steel fabricator or a building under construction and record the sections, connections and coatings you see.
• Find three examples in your city of corrosion on buildings and identify the cause, such as galvanic action, trapped water or salt.
• Calculate the thermal movement of a metal element on your community hub and show how the detail allows for it.
• Choose the fire protection for an exposed steel column in the hub's commons and explain the choice.
• Draw a detail where two different metals meet and show how they are separated.